After last updates (6.27 and may be some earlier) i found, that my RB951G-2HnD is getting PPPoE session disconnect.
I blamed my ISP, but tech support did not found any issues on their line. So i changed ISP to the new one without PPPoE auth and found that router is also loosing some packets time to time on the new line. About 5 icmp (about 20 secontd) and then continue working.
To be honest, i could not even imagine that the router could fail on such basic stuff.
Problem gone after i downgraded firmware to 6.11 version.